Exhaust gas heat exchangers are known—in particular in motor vehicles—as exhaust gas heaters, so-called additional heaters or as exhaust gas coolers. DE-A 199 62 863 from the applicant has disclosed an exhaust gas heat exchanger which can be used in particular as an additional heater in motor vehicles with consumption-optimized engines, in order to compensate the heating heat deficit of said engines. The exhaust gas dissipates its heat via a tube bundle of exhaust gas tubes to the coolant which flows through the heating body of the vehicle heater. In the event that the exhaust gas heating is no longer required, an exhaust gas valve switches the exhaust gas flow such that it flows through a bypass duct which is thermally insulated from the exhaust gas tubes, by means of a partition which largely suppresses any heating effect. The exhaust gas tubes are thus arranged in a housing which is traversed by coolant, while the bypass duct is accommodated in a separate, thermally insulated space. An advantage of said solution is the integration of the exhaust gas tubes and bypass duct; a disadvantage is the space-consuming thermal insulation of the bypass duct.
DE-A 102 03 003 has disclosed an exhaust gas heat exchanger which can be used predominantly as an exhaust gas cooler in an exhaust gas recirculation system (EGR system). Here, exhaust gas is extracted from an exhaust gas line of the internal combustion engine, is cooled in the exhaust gas cooler by the coolant of the coolant circuit, and is supplied back to the engine for complete combustion. Here, too, a bypass duct is connected parallel to a tube bundle of exhaust gas tubes, with the exhaust gas flow being conducted either through the tube bundle for the purposes of cooling or through the bypass duct in the case of non-cooling.